1) Field of the Invention
This invention relates to an automatic transmission, and especially to a hydraulic control system for engaging or releasing frictional engagement elements in a gear shift mechanism to shift the automatic transmission.
2) Description of the Related Art
In an automatic transmission, it has heretofore been needed, in some instances, to simultaneously perform engagement and/or release of two frictional engagement elements, in other words, so-called gear change operations upon shifting between particular speed stages. In such instances, the torque of an output shaft suddenly drops due to tie-up or engine racing occurs due to an underlap, unless the timing of the operations is appropriately controlled. With a view toward avoiding such problems, it has heretofore been the common practice to arrange one-way clutches in parallel with the respective frictional engagement elements so that while using the one-way clutches for releasing the corresponding frictional engagement elements, the timing of the gear change operations is automatically well maintained by controlling hydraulic pressure only for the engaging operations.
On the other hand, elimination of the above one-way clutches is desirable so that the gear shift mechanism can be more compactly constructed. One example of such a compact construction is disclosed in Japanese Patent Application Laid-Open (Kokai) No. HEI 1-224553 for shifting between the 2nd speed and the 3rd speed (hereinafter abbreviated as "2-3 shift"). This applies equally to other shifts and the 3-4 shift is effected without one-way clutches. In this transmission, to precisely control gear change operations of the corresponding frictional engagement elements (the clutches in this example) for a 2-3 shift, a timing valve is arranged in a hydraulic control system so that feeding of hydraulic pressure to both the clutches and discharge of hydraulic pressure from both the clutches can be conducted by a single directional control valve. Further, to ensure the above shift even if the valve should become inoperative due to sticking or the like, the hydraulic pressure on each discharge side can still be drained separately through an oil line, which is arranged in parallel with the directional control valve, via a small-diameter orifice.
However, in the system disclosed in the above patent publication, there is obviously a limitation on the amount of hydraulic pressure which can be discharged through the small-diameter orifice so that the system cannot prevent tie-up in the event of a failure of the directional control valve. If such tie-up takes place, a large shift shock occurs and, moreover, the heat load (transmitted torque.times.revolution number) on the engaging frictional engagement elements (clutches) becomes extremely large, resulting in a substantial reduction of the durability.